The importance of preventing unauthorized copying by such phototransference techniques confidential and other meaningful information such as customer lists, formulae, plans, designs and signatures is well recognized.
Previous attempts at copy prevention have not proven entirely satisfactory. Such approaches have, for example, included printing the information on paper colored to provide insufficient contrast with the printed material when photocopied by xerographic or electrostatic copying techniques but legible to the naked eye in ambient white light. However, such approaches require specially dedicated paper and printing techniques as limitations are imposed on the colors of print and paper and the contrast is usually considered to be rather poor so as to render the document somewhat difficult to read, particularly at a distance, unattractive to both read and handle and generally aesthetically undesirable.
Another approach has been to print the information in a color, such as yellow, to which the standard copier is relatively insensitive.
Examples of such prior approaches are taught by U.S. Pat. No. 4,632,429 issued Dec. 30, 1986 to Gardner; U.S. Pat. No. 4,522,429 issued Jun. 11, 1985 to Gardner and U.S. Pat. No. 4,281,931 issued Aug. 4, 1981 to Van Auken.
It is known to provide displays in which two or more pictures or words can be seen by varying the viewing angle, often to provide a motion or a three dimensional effect, by splitting representations of the different pictures or words into striae, arranged alternately in association with optical screen structures which provide preferential reflection of the respective striae at the differing angles of incident light. However, none of these proposals teach the present invention.
Examples of optical screen structures used for this purpose, including some which incorporate lenticular screens, are taught in U.S. Pat. No. 3,119,195 issued Jan. 28, 1964 to Broaunhut ; U.S. Pat. No. 3,586,592 issued Jun. 22, 1971 to Cahn; U.S. Pat. No. 1,475,430 issued Nov. 27, 1923 to Curwen; U.S. Pat. No. 3,268,238 issued Aug. 23, 1966 to Finkel; U.S. Pat. No. 2,832,593 issued Sep. 25, 1957 to Anderson and U.S. Pat. No. 1,969,551 issued Aug. 7, 1934 to Francis, the disclosures of which are incorporated by reference herein.